Spinal muscular atrophy (SMA), in its broadest sense, describes a collection of inherited and acquired central nervous system (CNS) diseases characterized by progressive motor neuron loss in the spinal cord and brainstem causing muscle weakness and muscle atrophy. The most common form of SMA is caused by mutations in the Survival of Motor Neuron (SMN) gene and manifests over a wide range of severity affecting infants through adults (Crawford and Pardo, Neurobiol. Dis., 1996, 3:97).
Infantile SMA is the most severe form of this neurodegenerative disorder. Symptoms include muscle weakness, poor muscle tone, weak cry, limpness or a tendency to flop, difficulty sucking or swallowing, accumulation of secretions in the lungs or throat, feeding difficulties, and increased susceptibility to respiratory tract infections. The legs tend to be weaker than the arms and developmental milestones, such as lifting the head or sitting up, cannot be reached. In general, the earlier the symptoms appear, the shorter the lifespan. As the motor neuron cells deteriorate, symptoms appear shortly afterward. The severe forms of the disease are fatal and all forms have no known cure. Infants with a severe form of SMA frequently succumb to respiratory disease due to weakness in the muscles that support breathing. Individuals with milder forms of SMA live much longer, although they may need extensive medical support, especially those at the more severe end of the spectrum. The clinical spectrum of SMA disorders has been divided into the following five groups.                (a) Type 0 SMA (In utero SMA) is the most severe form of the disease and begins before birth. Usually, the first symptom of Type 0 SMA is reduced movement of the fetus that can first be observed between 30 and 36 weeks of pregnancy. After birth, these newborns have little movement and have difficulties with swallowing and breathing.        (b) Type 1 SMA (Infantile SMA or Werdnig-Hoffmann disease) presents symptoms between 0 and 6 months. form of SMA is also very severe. Patients never achieve the ability to sit, and death usually occurs within the first 2 years without ventilatory support.        (c) Type 2 SMA (Intermediate SMA) has an age of onset at 7-18 months. Patients achieve the ability to sit unsupported, but never stand or walk unaided. Prognosis in this group is largely dependent on the degree of respiratory involvement.        (d) Type 3 SMA (Juvenile SMA or Kugelberg-Welander disease) is generally diagnosed after 18 months. Type 3 SMA individuals are able to walk independently at some point during their disease course but often become wheelchair-bound during youth or adulthood.        (e) Type 4 SMA (Adult onset SMA). Weakness usually begins in late adolescence in the tongue, hands, or feet, then progresses to other areas of the body. The course of adult SMA is much slower and has little or no impact on life expectancy.        
The SMN gene has been mapped by linkage analysis to a complex region in chromosome 5q (Lefebvre S. et al., Cell (1995) 80:155). In humans, this region contains an approximately 500 thousand base pairs (kb) inverted duplication resulting in two nearly identical copies of the SMN gene. SMA is caused by an inactivating mutation or deletion of the telomeric copy of the gene (SMN1) on both chromosomes, resulting in the loss of SMN1 gene function. However, all patients retain the centromeric copy of the gene (SMN2), and the copy number of the SMN2 gene in SMA patients generally correlates inversely with the disease severity; i.e., patients with less severe SMA have more copies of SMN2. SMN2 pre-mRNA undergoes alternative splicing of exon 7 caused by a translationally silent C to T mutation in exon 7. As a result, the majority of transcripts produced from SMN2 lack exon 7 (Δ7 SMN2), and encode a truncated SMN protein that has an impaired function and is rapidly degraded.
The SMN protein plays a role in RNA processing and metabolism, having a well characterized function of mediating the assembly of a specific class of RNA-protein complexes termed snRNPs. SMN may have other functions in motor neurons, however its role in preventing the selective degeneration of motor neurons is not well established.
In most cases, SMA is diagnosed based on clinical symptoms and by the complete absence of exon 7 of SMN1 gene. However, in approximately 5% of cases SMA is caused by mutation in genes other than the inactivation of SMN 1, some known and others not yet defined. In some cases, when the SMN 1 gene test is not feasible or does not show any abnormality, other tests such as an electromyography (EMG) or muscle biopsy may be indicated.
Medical care for SMA patients at present is limited to supportive therapy including respiratory, nutritional and rehabilitation care; there is no drug known to address the underlying cause of the disease. Current treatment for SMA consists of prevention and management of the secondary effects of chronic motor unit loss. The major management issue in Type 1 SMA is the prevention and early treatment of pulmonary problems, which are the cause of death in the majority of the cases. While some infants afflicted with SMA grow to be adults, those with Type 1 SMA have a life expectancy of less than two years.
Several mouse models of SMA have been developed. In particular, the SMN delta exon 7 (Δ7 SMN) model (Le et al., Hum. Mol. Genet., 2005, 14:845) carries both the SMN2 gene and several copies of the Δ7 SMN2 cDNA and recapitulates many of the phenotypic features of Type 1 SMA. The Δ7 SMN model can be used for both SMN2 expression studies as well as the evaluation of motor function and survival. The C/C-allele mouse model (Jackson Laboratory strain #008714, The Jackson Laboratory, Bar Harbor, Me.) provides a less severe SMA disease model but does not have an overt muscle weakness. This mouse model has the murine Smn1 gene inactivated and carries both the complete SMN2 gene and a hybrid mSmn1-SMN2 gene that undergoes alternative splicing. C/C-allele mice have reduced levels of both SMN2 full length (FL SMN2) mRNA and SMN protein. The C/C-allele mouse model is used for SMN2 expression studies.
As a result of improved understanding of the genetic basis and pathophysiology of SMA, several strategies for treatment have been explored, but none have yet demonstrated success in the clinic.
Gene replacement of SMN1, using viral delivery vectors, and cell replacement, using differentiated SMN1+/+ stem cells, have demonstrated efficacy in animal models of SMA. More research is needed to determine the safety and immune response and to address the requirement for the initiation of treatment at the neonatal stage before these approaches can be applied to humans.
Correction of alternative splicing of SMN2 in cultured cells has also been achieved using synthetic nucleic acids: (i) antisense oligonucleotides that target sequence elements in SMN2 pre-mRNA and shift the outcome of the splicing reaction toward the generation of full length SMN2 mRNA (Passini et al., Sci. Transl. Med., 2011, 3: 72ra18; and, Hua et al., Nature, 2011, 478:123) and (ii) trans-splicing RNA molecules that provide a fully functional RNA sequence that replace the mutant fragment during splicing and generate a full length SMN1 mRNA (Coady and Lorson, J Neurosci., 2010, 30:126).
Other approaches under exploration include searching for drugs that increase SMN levels, enhance residual SMN function, or compensate for its reduced levels. Aminoglycosides have been shown to enhance the expression of a stabilized extended Δ7 SMN protein produced from Δ7 SMN2 mRNA by promoting the translational read-through of the aberrant stop codon, but have poor central nervous system penetration and are toxic after repeat dosing.
Chemotherapeutic agents, such as aclarubicin, have been shown to increase SMN protein in cell culture; however, the toxicity profile of these drugs prohibits long-term use in SMA patients. Some drugs under clinical investigation for the treatment of SMA include transcription activators such as histone deacetylase (HDAC) inhibitors (e.g., butyrates, valproic acid, and hydroxyurea), and mRNA stabilizers (mRNA decapping inhibitor RG3039 from Pfizer), the goal being to increase the amount of total RNA transcribed from the SMN2 gene. However, the use of the HDAC inhibitors or mRNA stabilizers does not address the underlying cause of SMA and may result in a global increase in transcription and gene expression with potential safety problems in humans.
In an alternative approach, neuroprotective agents such as Olesoxime have been chosen for investigation. Such strategies are not aimed at SMN for the treatment of SMA, but instead are being explored to protect the SMN-deficient motor neurons from neurodegeneration.
A system designed for identifying compounds that increase the inclusion of exon 7 of SMN into RNA transcribed from the SMN2 gene and certain benzooxazole and benzoisoxazole compounds identified thereby have been described in International Patent Application WO2009/151546A1. A system designed for identifying compounds that cause ribosomal frameshifting to produce a stabilized SMN protein from Δ7 SMN2 mRNA and certain isoindolinone compounds identified thereby have been described in International Patent Applications WO2010/019236A1 and WO2013/119916A2.
Despite the progress made in understanding the genetic basis and pathophysiology of SMA, there remains a need to identify compounds that alter the course of spinal muscular atrophy, one of the most devastating childhood neurological diseases.